This invention relates generally to metal oxide semiconductor field effect transistors.
Metal oxide semiconductor field effect transistors include a gate that is self-aligned with a source/drain. The source/drain may include a deeper or heavily doped region and a shallower and lightly doped region, sometimes called a tip or source/drain extension.
Gate underlap is the amount by which the source/drain material diffuses under the gate after ion implantation and subsequent heat processing. After implantation, the material that is implanted is exposed to heat which causes the material to move downwardly into the substrate and, to a lesser extent, laterally under the gate. Thus, in a system using an ion implanted source/drain extension, the amount of underdiffusion is determined as a function of junction depth.
It is desirable to have relatively shallow junction depth for the source/drain extension to support smaller transistor dimensions. Usually, in source/drain extension implantation techniques, the minimum tip junction depths are determined by the necessary gate underlap.
The shallower the source/drain extension, generally the shorter the gate lengths that may be utilized without increasing off-state leakage currents. Extension doping under the gate edge is needed to ensure a low resistance path between the inversion layer under the gate and the highly doped source/drain extension region. The low resistance is needed for a high drive currents, which are critical for high circuit switching speeds.
Thus, there is a need for better ways to make source/drain junctions of field effect transistors.